selftests/bpf: Tests for uninitialized stack reads

Three testcases to make sure that stack reads from uninitialized
locations are accepted by verifier when executed in privileged mode:
- read from a fixed offset;
- read from a variable offset;
- passing a pointer to stack to a helper converts
STACK_INVALID to STACK_MISC.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230219200427.606541-3-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>